1. Field of the Invention
The present invention relates to cylindrical magnet systems as used in imaging systems such as MRI (Magnetic Resonance Imaging) systems.
2. Description of the Prior Art
FIG. 1 shows a radial cross-section through a typical magnet system for use in an imaging system. A cylindrical magnet 10, typically comprising superconducting coils mounted on a former or other mechanical support structure, is positioned within a cryostat, comprising a cryogen vessel 12 containing a quantity of liquid cryogen 15, for example helium, which holds the superconducting magnet at a temperature below its transition temperature. The cryogen vessel 12 is itself cylindrical, having an outer cylindrical wall 12a, an inner cylindrical bore tube 12b, and substantially planar annular end caps (not visible in FIG. 1). An outer vacuum container (OVC) 14 surrounds the cryogen vessel. It also is itself cylindrical, having an outer cylindrical wall 14a, an inner cylindrical bore tube 14b, and substantially planar annular end caps (not visible in FIG. 1). A hard vacuum is provided in the volume between the OVC 12 and the cryogen vessel 14, providing effective thermal insulation. A thermal radiation shield 16 is placed in the evacuated volume. This is typically not a fully closed vessel, but is essentially cylindrical, having an outer cylindrical wall 16a, an inner cylindrical bore tube 16b, and substantially planar annular end caps (not visible in FIG. 1). The thermal radiation shield 16 serves to intercept radiated heat from the OVC 14 before it reaches the cryogen vessel 12. The thermal radiation shield 16 is cooled, for example by an active cryogenic refrigerator 17, or by escaping cryogen vapor.
In alternative arrangements, the magnet is not housed within a cryogen vessel, but is cooled in some other way: either by a low cryogen inventory arrangement such as a cooling loop, or a ‘dry’ arrangement in which a cryogenic refrigerator is thermally linked to the magnet. There is no cryogen reservoir to absorb heat generated by ohmic heating of various conductive components by eddy currents.
The OVC bore tube 14b must be mechanically strong and vacuum tight, to withstand vacuum loading both radially and axially. Conventionally, it made of stainless steel. The cryogen vessel bore tube 12b, if any, must be strong and capable of withstanding the pressure of cryogen gas within the cryogen vessel. Typically, this is also of stainless steel. The bore tube 16b of the thermal radiation shield 16 must be impervious to infra-red radiation, and must be thermally and electrically conductive to provide electromagnetic shielding of the magnet from the gradient coils. It is preferably lightweight. It is typically made of aluminum.
The present invention may be applied in all such cases.
In order to provide an imaging capability, a set of gradient coils (not shown) are provided within the bore of the superconducting magnet. These are usually arranged as a hollow cylindrical, resin-impregnated block, containing coils which generate orthogonal magnetic field gradients in three dimensions.
During an imaging procedure, the gradient coils generate rapidly oscillating magnetic fields, for example at a frequency of about 1500 Hz. Stray fields from the gradient coils generate eddy currents in metal parts of the cryostat, in particular in metal bore tubes 14b, 16b, 12b of OVC, thermal shield and cryogen vessel, and also in the structure of the magnet 10. The eddy currents produced in the material of the OVC 14 will help to shield the thermal shield 16 and cryogenically cooled components such as cryogen vessel bore tube 12b, magnet coils and magnet former 10 from stray fields from the gradient coils. However, because of the constant background magnetic field produced by the magnet, those eddy currents produce Lorentz forces, acting radially and resulting in mechanical vibrations in the bore tube of the OVC.
These mechanical vibrations, in the constant background magnetic field of the magnet 10, will in turn cause secondary eddy currents in adjacent conductive materials, such as the bore tube 16b of the thermal radiation shield, or the bore tube 12b of a cryogen vessel. The secondary eddy currents will of course generate magnetic fields, known as secondary magnetic fields. These will interfere with imaging, and produce mechanical vibrations and secondary stray fields, which can be much larger than the stray fields produced by the gradient coils, in that region. The secondary stray fields also induce tertiary eddy currents in nearby conductive surfaces. These tertiary eddy currents will, in turn, generate tertiary magnetic fields, and so on.
A particular difficulty arises when, as is typical, the frequency of oscillation of the gradient magnetic fields is close to the resonant frequency of the bore tubes. It is known that a number of concentric tubes of similar diameters, such as the bore tubes of the OVC, thermal radiation shield and cryogen vessel of a typical MRI system, have similar effective resonant frequencies.
The mechanical vibrations will be particularly strong when a resonant vibration frequency of a bore tube corresponds to the frequency of oscillation of the stray field. If the resonant frequencies of the OVC, thermal shield, cryogen vessel if any, and magnet components are close together, as is the case in present magnets, the bore tubes behave as a chain of closely coupled oscillators, and resonance bands will occur.
The oscillations may also interfere with the imaging process, causing detriment to the resulting images.
The resulting oscillations cause acoustic noise which is most unpleasant for a patient in the bore, as well as interfering with imaging and causing heating of cooled components such as the thermal radiation shield and cryogen vessel, if any.
The eddy currents induced in the cryogenically cooled components of the magnet constitute an ohmic heat load on the cryogenic cooling system, leading to an increased consumption of liquid cryogen where used, or an increased heat load on the cryogenic refrigerator. In dry magnets—those which are not cooled by a liquid cryogen—the increased heat load can result in a temperature rise of the coils, which can result in a quench.
In U.S. Pat. No. 6,707,302, suggestions are made to adjust the resonant frequencies of bore tubes, by a number of alternative arrangements: choice of differing materials, such as aluminum or copper; corrugating the bore tubes; or slitting the bore tubes. Among these ideas, the choice of material has only a limited scope for changing resonant frequencies; corrugating of bore tubes consumes significant radial space within the magnet, leading to a more cramped patient bore, or the need to make larger diameter coils, which in turn significantly increases cost. Corrugations also introduce a high anisotropy: the resultant bore tube will be stiff in one direction, but flexible in a perpendicular direction. Slitting the bore tube worsens the shielding effect of the thermal radiation shield, and is not applicable to OVC or cryogen vessel bore tubes.